Studies will be carried out to determine the organization and regulation of expression of genes on yeast mitochondrial DNA. In particular, the genetic determinants for the variable size protein of the 37S mitochondrial ribosomal subunit, termed varl, will be investigated. A regulatory region on the mitochondrial genome has been identified which determines the apparent molecular weight of varl polypeptide. From DNA sequences studies, it appears that this region does not encode the protein. Experiments will be undertaken to determine the location of the varl structural gene and to determine the molecular mechanism by which its expression is controlled by a separate genetic regulatory element.